1. Field of the Invention
This invention relates to improvements in a rotating structure of a supercharger of the type wherein a ceramic turbine wheel is joined with a metallic rotatable shaft in a manner so that a projection of the turbine wheel is fitted in a cup-shaped section of the rotatable shaft and wherein a compressor wheel is installed to be biased against the cup-shaped section through an inner race of a rolling bearing for rotatably supporting the rotatable shaft under a screw torque of a nut, and more particularly to the rotating structure which is suitable for use in a turbocharger or a gas turbine.
2. Description of the Prior Art
A turbocharger is well known as an example of a supercharge in which intake air is supplied under pressure to a combustion chamber under the effect of energy developed by combustion. It is also well known that a turbine wheel of the turbocharger is made of a ceramic to improve a response in rotational movement. The ceramic turbine wheel is usually axially joined with a metallic rotatable shaft, which is accomplished, for example, by a shrinkage fit. In this shrinkage fit, an axial projection of the turbine wheel is fitted in a cup-shaped section formed at an end of the rotatable shaft under heating, in which the turbine wheel and the rotatable shaft are rigidly joined by virtue of a thermal shrinkage of the cup-shaped section. Additionally, it is well known that a bearing structure for rotatably supporting the rotatable shaft has been changed from a floating metal bearing to a rolling bearing in order to improve the response at a low vehicle speed.
A conventional turbocharger including the above-discussed known structure in connection with the rotatable shaft is shown in FIG. 4. In FIG. 4, the rotatable shaft 106 is rotatably supported by two rolling bearings 101, 102 which are located respectively at the sides of the turbine wheel 103 and the compressor wheel 104. A spacer 105 is disposed between the rolling bearings 101, 102. The inner races 101a, 102a of the rolling bearings 101, 102, the spacer 105 and the compressor wheel 104 are put between the cup-shaped section 108 and a nut 107 screwed on a compressor wheel side end portion of the rotatable shaft, so that they are fixedly mounted on the rotatable shaft like a single member.
However, the following drawbacks have been encountered in the above-discussed conventional turbocharger: The screw torque of the nut 107 develops a biasing force for urging the bearing inner race 101a against the cup-shaped section in a direction indicated by an arrow D, and simultaneously a tensile force for drawing the rotatable shaft 106 in a direction indicated by an arrow E, as shown in FIG. 4. In other words, a stress is developed at a position F between a location (to which the bearing inner race 101a is biased) of the cup-shaped section 108 and a major part of the rotatable shaft 106 due to the screw torque of the nut 107. If the screw torque of the nut 107 is increased, the bottom portion of the cup-shaped section 108 is drawn by the rotatable shaft 106, and therefore the cup-shaped section 108 deforms so that the rotatable shaft 106 moves toward the nut 107.
Thus, even if the screw torque of the nut 107 is increased, a sufficient force for clamping the two bearing inner races 101a, 102a, the spacer 105 and the compressor wheel 104 cannot be obtained between the cup-shaped section and the nut 107. Additionally, according to a particular distance between the bottom inner surface of the cup-shaped section 108 and the location (to which the bearing inner race 101a is biased) of the cup-shaped section 108, there is the possibility of weakening a holding force of the cup-shaped section 108 to the turbine wheel 103. The thus weakened holding force unavoidably causes the turbine wheel 103 to produce a change in rotational balance.